CHEMISTRY TEXTBOOK

anhydrous

FeCl 3 Cl^2

Cl

+ Cl 2

(Chlorobenzene)

+

Cl

Cl

(1,4 - Dichlorobenzene)

(major)

(1,2 - Dichlorobenzene)

(minor)

+ HCl

Cl

Cl

Remember...

The -I effect of Cl is more

powerful than its +R effect.

Therefore Cl is o-/p- directing but ring

deactivating group.

Can you recall?

• What is an electrophile?


• Give some examples of
  electrophiles


• What type of reactions are observed in
  benzene?

c. Electrophilic substitution (SE) in
arylhalides

Aryl halides undergo electrophilic

substitution reaction slowly as compared to

benzene.

In resonance structures of chlorobenzene

(see section 10.6.5) elelctron density is

relatively more at ortho and para position.

Therefore incoming electrophilic group is

more likely to attack at these positions. But

due to steric hinderance at ortho position, para

product usually predominates. In haloarenes,

halogen atom has strong electron withdrawing

inductive effect (-I). This deactivates the ring

and electrophilic substitution reaction occurs

slowly.

Do you know?

Occurrence of nucleophilic

substitution in p-nitrochlorobenzene

can be explained on the basis of resonance

stabilization of the intermediate.

Cl

OH

O O

N

⊕

+

(I)

Cl OH

O O

N

⊕

slow

step

Cl OH

O O

N

⊕

(III)

Cl OH

O O

N

⊕

(II)

OH

(IV)

Cl

O O

N

⊕

fast

step

OH

O O

N

+Cl

⊕

The resonance structure (III) shows

that the electron withdrawing nitro group

(-NO 2 ) in the p-position extends the

conjugation. As a result, the intermediate

carbanion is better stabilized which favours

nucleophilic substitution reaction.

• Identify the product A of following
  reaction.

+ HNO 3 conc. H^2 SO^4 A

i. Halogenation : It is carried out by reacting

haloarene with halogen in presence of ferric

salt as Lewis acid catalyst.